Summer Heatwave Impacts on the European Kelp Saccharina latissima Across Its Latitudinal Distribution Gradient

Abstract

Kelps are important foundation species in coastal ecosystems currently experiencing pronounced shifts in their distribution patterns caused by ocean warming. While some populations found at species’ warm distribution edges have been recently observed to decline, expansions of some species have been recorded at their cold distribution edges. Reduced population resilience can contribute to kelp habitat loss, hence, understanding intraspecific variations in physiological responses across a species’ latitudinal distribution is crucial for its conservation. To investigate potential local responses of the broadly distributed kelp Saccharina latissima to marine heatwaves in summer, we collected sporophytes from five locations in Europe (Spitsbergen, Bodø, Bergen, Helgoland, Locmariaquer), including populations exposed to the coldest and warmest local temperature regimes. Meristematic tissue from sporophytes was subjected to increasing temperatures of Δ+2, Δ+4 and Δ+6°C above the respective mean summer temperatures (control, Δ±0°C) characteristic for each site. Survival and corresponding physiological and biochemical traits were analyzed. Vitality (optimum quantum yield, Fv/Fm) and growth were monitored over time and biochemical responses were measured at the end of the experiment. Growth was highest in northern and lowest in southern populations. Overall, northern populations from Spitsbergen, Bodø and Bergen were largely unaffected by increasing summer temperatures up to Δ+6°C. Conversely, sporophytes from Helgoland and Locmariaquer were markedly stressed at Δ+6°C: occurrence of tissue necrosis, reduced Fv/Fm, and a significantly elevated de-epoxidation state of the xanthophyll cycle (DPS). The variations in phlorotannins, mannitol and tissue C and N contents were independent of temperature treatments and latitudinal distribution pattern. Pronounced site-specific variability in response to increasing temperatures implies that exceeding a threshold above the mean summer temperature exclusively affect rear-edge (southernmost) populations.

Seasonal and inter‐annual variability in the heatwave tolerance of the kelp Saccharina latissima (Laminariales, Phaeophyceae)

“…The experimental set‐up was based on the temperature‐amplitude experiment by Diehl et al (2021a). Meristematic discs (diameter 2.2–2.4cm) were cut and afterwards equally distributed between treatments and replicates ( n = 4), avoiding pseudo‐replication, and cultivated in aerated 2 L clear‐plastic bottles, filled with sterile ½ Provasoli enriched seawater (½ PES) (Provasoli 1968) with the following modifications: Hepes‐buffer instead of Tris, double concentration of Na 2 glycerophosphat, iodine enrichment after Tatewaki 1966; Salinity (S A ) = 33, applying a 16:8 h light:dark photocycle of 30–35 μmol photons m −2 s −1 (ProfiLux 3 with LED Mitras daylight 150; GHL Advanced Technology, Kaiserslautern, Germany).…”

“…Meristematic discs (diameter 2.2–2.4cm) were cut and afterwards equally distributed between treatments and replicates ( n = 4), avoiding pseudo‐replication, and cultivated in aerated 2 L clear‐plastic bottles, filled with sterile ½ Provasoli enriched seawater (½ PES) (Provasoli 1968) with the following modifications: Hepes‐buffer instead of Tris, double concentration of Na 2 glycerophosphat, iodine enrichment after Tatewaki 1966; Salinity (S A ) = 33, applying a 16:8 h light:dark photocycle of 30–35 μmol photons m −2 s −1 (ProfiLux 3 with LED Mitras daylight 150; GHL Advanced Technology, Kaiserslautern, Germany). Meristematic discs are commonly used to investigate physiological stressors on kelps (Liesner et al 2020a; Diehl et al 2021a). To minimize the physical impact of the disc cutting, a 3‐day wound‐healing phase at 18°C was conducted prior to the experiment (Fig.…”

“…Sporophytes were collected on the 26 June 2018 (54 10.9 0 N, 7 55.1 0 E). The experiment was conducted by Diehl et al (2021a). Raw data was downloaded from the PANGAEA data platform (Diehl et al 2021b) and re-evaluated for this study.…”

“…The optimum growth temperature of S. latissima sporophytes usually ranges between 10 and 15°C (Fortes and Lüning 1980; Lüning 1979) and the lethal temperature was determined at 23°C (Bolton and Lüning 1982). However, Diehl et al (2021a) detected that specimens from Locmariaquer (Brittany, France) can even survive temperatures up to 25°C for more than 1 week after successive temperature increase, mimicking MHW conditions.…”

“…The experiments were run in August 2018 and August 2019 to investigate inter‐annual differences. Additionally, seasonal effects were evaluated by comparing the data from June 2018 (Diehl et al 2021a) with the results from August 2018. We hypothesized that physiological and biochemical responses of S. latissima to heat stress are affected by the local temperature history and, thus, vary between seasons and years.…”